1. Field of the Invention
The present invention relates to a hydrogen storage material capable of reversibly storing and releasing hydrogen, and a method for producing the same.
2. Description of the Related Art
Fuel-cell cars are equipped with fuel cells for generating electric power utilizing an electrochemical reaction between hydrogen and oxygen. Thus, a motor of the fuel-cell car is actuated by the electric power from the fuel cell to generate driving force for rotating wheels.
The oxygen can be obtained from the air, and the hydrogen is generally supplied from a hydrogen storage vessel. Therefore, the fuel-cell car is further equipped with the hydrogen storage vessel.
As the hydrogen storage vessel has a higher hydrogen storage capacity, the fuel-cell car can be driven over a longer distance. However, when the fuel-cell car contains an excessively large gas storage vessel, the weight of the fuel cell car is increased, resulting in a higher load on the fuel cell disadvantageously. From this viewpoint, various techniques have been studied in order to acquire a hydrogen storage vessel having a high hydrogen storage capacity with a small volume. In one of these techniques, a hydrogen storage material is placed inside the hydrogen storage vessel. For example, in Japanese Laid-Open Patent Publication No. 2004-018980, AlH3, which can store a great amount of hydrogen, i.e., 10% by weight of its own weight, is reported as an effective hydrogen storage material.
As shown in FIG. 16, a crystalline AlH3 1 has a microstructure containing matrix phases 2 approximated by squares and a grain boundary phase 3 disposed between the matrix phases 2, 2. In this case, the matrix phases 2 have a side length t1 of approximately 100 μm, and the grain boundary phase 3 has a width w1 of several micrometers and occupies only a several volume percent of the structure. In an X-ray diffraction measurement of the crystalline AlH3, sharp peaks of at least one of α, β, and γ phases can be observed in the diffraction pattern.
It should be noted that the matrix phases 2 are composed of AlH3 having a crystal lattice containing Al and H, and the grain boundary phase 3 is composed of a solid solution of H in an amorphous Al.
In the crystalline AlH3 1, hydrogen is stored in accordance with the following formula (1), while the stored hydrogen is released in accordance with the formula (2). The formulae (1) and (2) represent reactions at an arbitrary storage/release site, and do not mean that all sites of the crystalline AlH3 1 are oxidized and reduced.Al+3/2H2→AlH3  (1)AlH3→Al+3/2H2  (2)
It is known that the release reaction represented by the formula (2) can relatively readily proceed, but the storage reaction represented by the formula (1) cannot readily proceed. For example, as described in Japanese Laid-Open Patent Publication No. 2004-018980, hydrogen gas can be stored again (re-stored) when the AlH3 is doped with Ti and NaH and then ball-milled under a hydrogen pressure of 100 atm.
In addition, as described in Sergei K. Konovalov and Boris M. Bulychev, Inorganic Chemistry, 1995, 34, pp 172-175 (particularly page 173, right column, lines 26-28 and FIG. 2), when Al is hydrogenated by H2 gas contact in a gas-phase process, the hydrogenation has to be carried out under a higher pressure of more than 2.5 GPa (about 25000 atm) at 280° C. through 300° C. or under a further higher pressure of 4 through 6 GPa at 450° C. through 550° C.
As described above, the crystalline AlH3 is disadvantageous in that it cannot readily store the hydrogen.